The present invention relates to a thread cutting machine, and particularly to one which has a hook arm movable to hook and retreat threads for a cutting knife to cut off; the hook rod is fitted to a hook rod arm which can be adjusted in respect of position relative to a swing arm for adjusting the moving path of the hook rod.
Related prior arts of the thread cutting device for a sewing machine of the present invention include U.S. Pat. Nos. 4,098,209 and 2, 892,012; 5,481,994 was also devised by the inventor of the present invention, aimed at improving U.S. Pat. No. 4,098,209.
Referring to FIG. 10, a conventional thread cutting device of a sewing machine has fixed plate 61, a cutting knife arm 62, a securing member 63, a cutting knife 64, a hook rod arm 65, a hook rod 66 and a swing arm 67.
The fixed plate 61 has several fixing holes 611, and is fixed to the sewing machine by means of screws 6111 passed through the fixing holes 611. The fixed plate 61 further has a receiving recess 612 and screw holes 613, 6121.
The cutting knife arm 62 has a through hole 621, a guide slot 622, screw holes 623, 623 at an intermediate portion, and a screw hole 624 at a front end portion. The cutting knife arm 62 is pivoted to the fixed plate 61 by a screw 6212 and an axle cap 6211; the screw 6212 being passed through the through hole 621 and the a-de cap 6211, and connected to the screw hole 6121 on the receiving recess 612; the rear end portion of the cutting knife arm 62 being received in the receiving recess 612.
The cutting knife 64 is fixed to the front end portion of the cutting knife arm 64 with a screw 642 screwed into the screw hole 624; a pad 643 is disposed between the cutting knife 64 and the cutting knife arm 62.
The securing member 63 is also fixed to the front end portion of the cutting knife arm 62 by screws 632 with a fixing plate 633 connected to a bottom.
The hook rod arm 65 has a gap at a rear end portion, two elongated guide holes 652 and screw holes 653 at a front end portion. The hook rod arm 65 is movably connected to the cutting knife arm 62 by means of two screws 6522 each passed through an axle cap 6521, a respective one of the elongated guide holes 652, and screwed into a respective one of the screw holes 623 of the cutting knife arm 62.
The hook rod 66 is connected to the front end portion of the hook rod arm 65 by screws 662 screwed into the screw holes 653. The hook rod 66 has a front hook 663 at a front end, and a rear hook 664 behind the front hook 663.
The swing arm 67 has a pivotal hole 671, a through hole 674 and a head 673. The swing arm 67 is pivoted to the fixed plate 61 by means of a screw 6721 passed through an axle cap 672, the pivotal hole 671 and the screw hole 613 of the fixed plate 61 in sequence. The head 673 of the swing arm 67 is movably received in the gap 651 of the hook rod arm 65 such that the hook rod arm 65 can be moved back and forth relative to the cutting knife arm 62 when the swing arm 67 is swung. Moreover, a screw pin 6751 is connected to the through hole 674 of the swing arm 67 and nuts 676, 677 from two ends. The nut 677 is movably received in the guide slot 622 of the cutting knife arm 62.
In using the thread cutting device, referring to FIG. 11, firstly, the swing arm 67 is swung for the head 673 to move forwards with the nut 677 moving from the curved rear end position to a straight front portion; thus, the cutting knife arm 62 will be pivoted on the screw 6212 to a position substantially parallel to lateral sides of the fixed plate 61. Meanwhile, the hook rod arm 65 is also moved to the parallel position.
Secondly, referring to FIG. 12, the swing arm 67 is swung further forwards, forcing the hook rod arm 65 to move forwards relative to the cutting knife arm 62. Thus, hook rod 66 projects from the cutting knife 64, and hook threads from the hooks 663, 664. Then, the swing arm 67 is swung back to retreat the hook rod arm 65; thus, the threads are cut by the cutting knife 64 and the hooks 663, 664 when the hooks 663, 664 move past the cutting knife 64; the hook rod 66 abutting the cutting knife 64 for the threads to be cut easily because of the securing member 63.
However, the above thread cutting device of a sewing machine is found to have disadvantages as follows.
1. The arm of force A, i.e. the distance between the fulcrum screw 6212 and the nut 677, is much shorter than B, i.e. the distance between the nut 677 and the hook rod 66. Consequently, relatively much force has to be applied to swing the swing arm 67, resulting in waste of power. PA1 2. Using the screw 6212 as a fulcrum, the front and the rear hooks 663, 664 of the hook rod 66 has relatively large range of oscillation, and cannot move very smoothly. Consequently, the service life of related parts can be shortened. PA1 3. Using the screw 6212 as a fulcrum, the cutting knife arm 62 and other parts associated with it cannot be adjusted to an optimum position.